How to make a drywall smooth?

I had a popcorn ceiling which I removed and now there is a drywall ( I think it is called so) and lots of tapes. I would need to make it smooth for painting it. I tried to put ready mixed filler on the tape and then sand it. The problem is that when I sand the edges of the area that I have put the filler on, it creates holes in the drywall. What should I do?